grepmonster

/*
 * ----------------------------------------------------------------------------
 * "THE BEER-WARE LICENSE" (Revision 42):
 * grepmonster.wordpress.com wrote this file. As long as you retain this notice you
 * can do whatever you want with this stuff. If we meet some day, and you think
 * this stuff is worth it, you can buy me a beer in return. Eddy
 * ----------------------------------------------------------------------------
 */

public class Halt
{
  public static void main(String [] args)
  {
    //If we are given arguments, convert them to an int and pass to f
    if (args.length > 0)
    {
      long input;

      for (String string: args)
      {
        try
        {
          input = Long.parseLong(string);

          if (input < 0)
          {
            throw new NumberFormatException();
          }

          f(input);
          System.out.println();
          System.out.println("--------------------------------------------------------------------------------");
        }
        catch (NumberFormatException e)
        {
          System.err.println("Argument must be a natural number");
          System.exit(1);
        }
      }
    }
    else
    {
        //Try all inputs starting from 0
      long input = 0;

      while (true)
      {
        f(input);
        System.out.println();
        System.out.println("--------------------------------------------------------------------------------");
        input++;
      }
    }
  } // main

  private static void f(long x)
  {
    System.out.print(x);

    while (x > 1)
    {
      //if x is even x := x / 2
      if (x % 2 == 0)
      {
        x /= 2;
      }
      //otherwise x := 3x + 1
      else
      {
        x = (3 * x) + 1;
      }

      System.out.print(", " + x);
    }

    System.out.print(", halt.");
  } //f
} //Halt